I'm sorry this is happening. Unfortunately I don't have anything to help you at this point. Regardless of what made them ill, the inability to control the neck and move in general are usually the last stage of an illness before they pass. You did your best to do right by him, and you're willing to give him a peaceful passing. Although it doesn't feel like that was enough, he was loved and wasn't alone, you will mourn him and care for his hen, and that's more than some people ever have.
The first Vet gave you the antibiotic in case it was an ear infection, you've been giving vitamins and feeding a complete feed (even if it was layer feed), all things that should have made him improve if it were most of the simple, common things that get mistaken for Marek's. What's left are toxicity or disease.
You are correct that she will be lonely. Other than spending time with her every day, I don't know of a lot of good boredom busters that don't involve food, maybe something low calorie/high water like a watermelon would be fun, but she doesn't need to put on any extra weight. You might have to tidbit for her to let her know you're the "rooster" taking care of her for a while. Mine could literally spend all day digging around in a pile of wood chips, that might be something to try if you've got the room. Many cities give away free mulch, but don't buy any dyed or cedar mulch at the store.
I'd wait at least a month before getting any other chickens, to see if she falls ill. Hens already laying are less likely to fall ill than chicks or pullets. Maybe there's an animal rescue in your area that knows of some adult birds in need of a home. We have some poultry-only rescues here, and the normal humane societies and animal shelters get chickens more often than you think. Other chicken people might be able to help out too. Just make sure they're not dumping their problems like whoever abandoned your pair. Don't be demanding, but don't be afraid to ask questions - any keeper/breeder worth their salt is happy to teach and make sure you're both comfortable given the circumstances. Until you do find out whether it's Marek's, try to stay away from other chickens and places that sell them, and be upfront that you might have had Marek's with anyone you discuss getting birds from. Don't be offended if someone that has vaccinated birds still wants to meet you away from their flock somewhere. They can send you video of the birds for you to look at. Do be prepared for lectures or even meanness, Marek's is ubiquitous nearly worldwide now, but most people don't know or understand that, and it's not worth your time to try to explain it to someone who attacks you.
Take care and good luck.
